Aroma characteristic and volatile profiling of carrot varieties and quantitative role of terpenoid compounds for carrot sensory attributes

Sensory Perception and Consumer Acceptance of Carrot Cultivars Are Influenced by Their Metabolic Profiles for Volatile and Non-Volatile Organic Compounds

Sensory parameters as well as the volatile and non-volatile compound profiles of sixteen carrot cultivars were recorded to obtain insight into consumer preference decisions. The sensory test was carried out with a consumer panel of 88 untrained testers allowing a clear acceptance-based differentiation of the cultivars. Five individual sensory characters (sweetness, overall aroma, bitterness, astringency and off-flavor) supported this discrimination. Chemical analyses of volatile organic compounds, polyacetylenes, phenylpropanoids and sugars enabled us to correlate the influence of these ingredients on sensory perception. Higher concentrations of α-pinene, hexanal, styrene and acetophenone correlated with a better acceptance, as well as sweetness and overall aroma perception. In contrast, a low acceptance as well as a stronger perception of bitterness, astringency and off-flavor correlated with enhanced concentrations of camphene, bornylacetate, borneol, myristicine, falcarindiol, falcarindiol-3-acetate, laserin and epilaserin. The present study should support the development of new breeding strategies for carrot cultivars that better satisfy consumer demands.

Comprehensive evaluation of aroma and taste properties of different parts from the wampee fruit

Wampee is a tropical fruit having high medicinal value. To fully realize the fruit's potential, it is essential to reveal the flavor characteristics. In this study, a comprehensive analysis of the aroma and taste profiles of different parts from the wampee fruit was conducted. The aroma profile was analyzed by E-nose, and 67 volatile components were identified through HS-SPME-GC-MS. Among them, 11 were considered as crucial compounds. Additionally, 42 volatile components were identified by HS-GC-IMS, with 22 compounds showing a variable importance in projection scores greater than 1.0. Moreover, the taste profile and representative compounds were analyzed by E-tongue and HPLC, and 12 compounds were considered as important taste contributors based on taste activity value. These findings shed light on the various compounds responsible for the unique aroma and taste of the wampee fruit, providing theoretical foundation for exploring ways for its comprehensive utilization and development.

The carrot monoterpene synthase gene cluster on chromosome 4 harbours genes encoding flavour-associated sabinene synthases

In plants, low molecular weight terpenes produced by terpene synthases (TPS) contribute to multiple ecologically and economically important traits. The present study investigates a carrot terpene synthase gene cluster on chromosome 4 associated with volatile monoterpene production. Two carrot mutants, yellow and cola, which are contrasting in the content of low molecular weight terpenes, were crossed to develop an F2 mapping population. The mapping analysis revealed overlapping QTLs on chromosome 4 for sabinene, α-thujene, α-terpinene, γ-terpinene, terpinen-4-ol and 4-carene. The genomic region of this locus includes a cluster of five terpene synthase genes (DcTPS04, DcTPS26, DcTPS27, DcTPS54 and DcTPS55). DcTPS04 and DcTPS54 displayed genotype- and tissue-specific variation in gene expression. Based on the QTL mapping results and the gene expression patterns, DcTPS04 and DcTPS54 were selected for functional characterization. In vitro enzyme assays showed that DcTPS54 is a single-product enzyme catalysing the formation of sabinene, whereas DcTPS04 is a multiple-product terpene synthase producing α-terpineol as a major product and four additional products including sabinene, β-limonene, β-pinene and myrcene. Furthermore, we developed a functional molecular marker that could discriminate carrot genotypes with different sabinene content in a set of 85 accessions.

Diversity and function of terpene synthases in the production of carrot aroma and flavor compounds

Carrot (Daucus carota L.) is an important root vegetable crop with high nutritional value, characteristic flavor, and benefits to human health. D. carota tissues produce an essential oil that is rich in volatile terpenes and plays a major role in carrot aroma and flavor. Although terpene composition represents a critical quality attribute of carrots, little is known about the biosynthesis of terpenes in this crop. Here, we functionally characterized 19 terpene synthase (TPS) genes in an orange carrot (genotype DH1) and compared tissue-specific expression profiles and in vitro products of their recombinant proteins with volatile terpene profiles from DH1 and four other colored carrot genotypes. In addition to the previously reported (E)-β-caryophyllene synthase (DcTPS01), we biochemically characterized several TPS proteins with direct correlations to major compounds of carrot flavor and aroma including germacrene D (DcTPS7/11), γ-terpinene (DcTPS30) and α-terpinolene (DcTPS03). Random forest analysis of volatiles from colored carrot cultivars identified nine terpenes that were clearly distinct among the cultivars and likely contribute to differences in sensory quality. Correlation of TPS gene expression and terpene metabolite profiles supported the function of DcTPS01 and DcTPS03 in these cultivars. Our findings provide a roadmap for future breeding efforts to enhance carrot flavor and aroma.

Headspace Volatile Evaluation of Carrot Samples-Comparison of GC/MS and AuNPs-hpDNA-Based E-Nose

The performances of a quartz crystal microbalances (QCMs) based on an electronic nose (E-nose), modified with hairpin-DNA (hpDNA) for carrot aroma profiling has been evaluated. Solid phase micro-extraction (SPME) headspace sampling, combined with gas chromatography (GC), was used as a reference method. The changes in carrot aroma profiles stored at different temperatures (−18 °C, 4 °C, 25 °C, and 40 °C) were monitored during time up to 26 days. The principal component analysis of the data evidenced the different aroma patterns related to the presence of different key compounds. The output data achieved with the hpDNA-based E-nose were able to detect aroma patterns similar to gas chromatography with mass spectrometry (GC-MS). This work demonstrates that hpDNA has different sizes of loops that can be used for the development of sensor arrays able to detect aroma patterns in food and their changes with advantages in terms of easiness of usage, rapidity, and cost of analysis versus classical methods.

Next-Gen Approaches to Flavor-Related Metabolism

Although flavor is an essential element for consumer acceptance of food, breeding programs have focused primarily on yield, leading to significant declines in flavor for many vegetables. The deterioration of flavor quality has concerned breeders; however, the complexity of this trait has hindered efforts to improve or even maintain it. Recently, the integration of flavor-associated metabolic profiling with other omics methodologies derived from big data has become a prominent trend in this research field. Here, we provide an overview of known metabolites contributing to flavor in the major vegetables as well as genetic analyses of the relevant metabolic pathways based on different approaches, especially multi-omics. We present examples demonstrating how omics analyses can help us to understand the accomplishments of historical flavor breeding practices and implement further improvements. The integration of genetics, cultivation, and postharvest practices with genome-scale data analyses will create enormous potential for further flavor quality improvements.

Biosynthesis of methyleugenol and methylisoeugenol in Daucus carota leaves: Characterization of eugenol/isoeugenol synthase and O-Methyltransferase

Carrot (Daucus carota subsp. sativus) is a widely cultivated root vegetable of high economic importance. The aroma of carrot roots and aboveground organs is mainly defined by terpenes. We found that leaves of orange carrot cultivar also produce considerable amounts of the phenylpropenes methyleugenol and methylisoeugenol. Notably, methyleugenol is most abundant in young leaves, while methylisoeugenol is the dominant phenylpropene in mature leaf tissue. The goal of the present study was to shed light on the biochemistry and molecular biology of these compounds' biosynthesis and accumulation. Using the available genomic and transcriptomic data, we isolated a cDNA encoding eugenol/isoeugenol synthase (DcE(I)GS1), an NADPH-dependent enzyme that converts coniferyl acetate to eugenol. This enzyme exhibits dual product specificity and yields propenylphenol isoeugenol alongside allylphenol eugenol. Furthermore, we identified a cDNA encoding S-adenosyl-L-methionine:eugenol/isoeugenol O-methyltransferase 1 (DcE(I)OMT1) that produces methyleugenol and methylisoeugenol via methylation of the para-OH-group of their respective precursors. Both DcE(I)GS1 and DcE(I)OMT1 were expressed in seeds, roots, young and mature leaves, and the DcE(I)OMT1 transcript levels were the highest in leaves. The DcE(I)GS1 protein is 67% identical to anise t-anol/isoeugenol synthase and displays an apparent K_m of 247 μM for coniferyl acetate. The catalytic efficiency of DcEOMT1 with eugenol is more than five-fold higher than that with isoeugenol, with K_m values of 40 μM for eugenol, and of 115 μM for isoeugenol. This work expands the current knowledge of the enzymes involved in phenylpropene biosynthesis and would enable studies into structural elements defining the regioselectivity of phenylpropene synthases.

Effect of Antifreeze Peptide Pretreatment on Ice Crystal Size, Drip Loss, Texture, and Volatile Compounds of Frozen Carrots

Ice crystal formation is of primary concern to the frozen food industry. In this study, the effects of antifreeze peptides (AFPs) on ice crystal formation were assessed in carrot during freezing and thawing. Three synthetic analogues based on naturally occurring antifreeze peptides were used in this study. The AFPs exhibited modification of ice crystal morphology, confirming their antifreeze activity in vitro. The ability of the synthetic AFPs to minimize drip loss and preserve color, structure, texture, and volatiles of frozen carrot was evaluated using the techniques of SEM, GC-MS, and texture analysis. The results prove the potential of these AFPs to preserve the above characteristics in frozen carrot samples.

Identification and Characterization of Terpene Synthases Potentially Involved in the Formation of Volatile Terpenes in Carrot (Daucus carota L.) Roots

Plants produce an excess of volatile organic compounds, which are important in determining the quality and nutraceutical properties of fruit and root crops, including the taste and aroma of carrots (Daucus carota L.). A combined chemical, biochemical, and molecular study was conducted to evaluate the differential accumulation of volatile terpenes in a diverse collection of fresh carrots (D. carota L.). Here, we report on a transcriptome-based identification and functional characterization of two carrot terpene synthases, the sesquiterpene synthase, DcTPS1, and the monoterpene synthase, DcTPS2. Recombinant DcTPS1 protein produces mainly (E)-β-caryophyllene, the predominant sesquiterpene in carrot roots, and α-humulene, while recombinant DcTPS2 functions as a monoterpene synthase with geraniol as the main product. Both genes are differentially transcribed in different cultivars and during carrot root development. Our results suggest a role for DcTPS genes in carrot aroma biosynthesis.